Eggs are nutritious, but are also very high in cholesterol. The cholesterol content is approximately 1.4 percent of the egg yolk weight. Furthermore, about one-third of the yolk weight is fat. Fatty acid analysis shows that the fat content of an average yolk is 35.4 percent saturated fat (principally palmitic and stearic acids), 49.1 percent monounsaturated fat (oleic acid), and 15.5 percent polyunsaturated fat (linoleic acid). Thus, the ratio of polyunsaturated fat to saturated fat is less than 1 to 2, which is an undesirable balance.
Substantial research efforts have been directed to the removal of cholesterol from egg yolks as well as food products containing egg yolks. A variety of approaches to remove cholesterol from egg yolks have been tried. One approach utilizes microorganisms to enzymatically oxidize and degrade cholesterol [Japanese Patent 60-18375], but this method produces cholesterol oxidation products which may be undesirable. Organic solvents have also been used to extract cholesterol [JP 48/44458, U.S. Pat. Nos. 4,104,286, 3,881,034, 3,563,765, 4,234,619 and Tokarska, et al., Can. Inst. Food Sci. Tech. J. 18:256-258 (1985)], but such methods remove a substantial portion of the triglyceride oils, and may involve significant processing steps to remove solvents. Even with supercritical carbon dioxide as the solvent, selectivity of cholesterol over triglycerides may be limited [Prepared Foods, 157:186 (1988); JP 59/135847].
U.S. Pat. No. 4,804,555 discloses a process for the simultaneous deodorization and cholesterol level reduction of fish oils. The fish oil is deaerated, mixed with steam, heated, flash vaporized, thin film-stripped with a countercurrent stream and cooled. U.S. Pat. No. 2,234,619 discloses a method for cholesterol removal from eggs by dehydrating the eggs, extracting the cholesterol with liquid dimethyl ether under anhydrous conditions and removing the dimethyl ether by treatment under reduced pressure at low temperatures. U.S. Pat. No. 4,104,286 describes the isolation of cholesterol from eggs through extraction with ethanol, saponification in an aqueous ethanolic alkali metal hydroxide and concentration and purification with a hydrocarbon solvent and methanol. U.S. Pat. No. 4,393,044 describes the formation of a steroid ester to be used as an oil phase material or base material for cosmetics or ointments.
Edible vegetable oils have also been used to extract cholesterol from egg yolks, as described in U.S. Pat. Nos. 3,717,414 and 4,103,040. However, such extraction using a vegetable oil to remove the cholesterol from yolk produces a waste stream of spent oil. In order to make this an economical process, further development is necessary to form a process for regenerating, rather than disposing this spent oil.
Another method of extracting cholesterol from eggs is disclosed in U.S. Pat. No. 4,333,959. The cholesterol is removed from the egg yolk by adding to the egg yolk an acid to reduce the pH of the egg yolk and then treating the egg yolk with an edible oil at a ratio of 3:5 oil to yolk to form a dispersion. The dispersion is centrifuged to form an egg-yolk phase and an oil phase. This method does not alleviate the problem of reducing the amount of oil necessary to extract the cholesterol; therefore, a considerable amount of oil is still necessary to extract the cholesterol, which thereby creates a problem of having to regenerate the oil containing the cholesterol.
Prior to this invention herein, a limiting factor in reducing the amount of oil was that emulsions were formed. For instance, even when the ratio of oil to egg yolk was 2:1, upon shearing the extraction mixture, oil-in-water emulsions were observed. Therefore, it would be highly desirable to limit the formation of an oil-in-water emulsion, while decreasing the amount of oil necessary for the extraction. It would also be desirable to provide egg products having desirable functional characteristics of egg yolk products (e.g., protein and phospholipids), while reducing the cholesterol level, increasing the polyunsaturated fat to saturated fat ratio, while reducing the amount of oil necessary to remove a substantial amount of the cholesterol. It would further be desirable to do this while retaining the functionality, appearance and taste of natural eggs which would yield good texture upon being cooked and the like.